The fiber-optic PO2 sensor instrumentation, which was developed previously, was used to evaluate the effect of oxygen partial pressure on the effectiveness of photodynamic therapy. Photodynamic therapy is being evaluated in clinical trials for several different tumors. The primary mechanism for cell-kill is believed to be the generation of singlet oxygen from in situ molecular oxygen. The objective of this study was to measure tumor tissue oxygen tension during PDT to ascertain whether significant hypoxia occurs; and, if so, whether tumor reoxygenation occurs after cessation of light.